1. Technical Field of the Invention
This invention relates to a submersible motor and seal section for a submersible motor. More particularly, it relates to a submersible motor, its ability to operate in air or submerged, and its maintainability. This invention particularly references combined submersible motor and pump units, although certain features of the present invention are useful on submersible motors that are used for purposes other than the operation of pumps.
2. Background Art
The term xe2x80x9csubmersiblexe2x80x9d, as used herein means that the motor can be surrounded by a fluid, which is restricted from access to the interior of the motor by an external casing or motor housing that is integral to the motor design.
Submersible motor driven pumps are widely used for transferring liquids from sumps and wells. Generally, these pumps include a motor, and a seal section that prevents the ingress of the pumped fluid along the motor shaft. Submersible motors have been designed with both wet and dry rotors. Wet rotor designs incorporate a rotor chamber filled with a compatible fluid to lubricate bearings and remove heat. The fluid must have good dielectric properties so that electrical conduction does not occur between the fluid and the motor windings.
There is, however, a disadvantage in the use of a fluid filled rotor chamber in that viscous drag due to the fluid properties will result in a decrease of the overall efficiency of the motor. This decrease in efficiency can become quite significant as the motor size increases.
Dry rotor designs have segregated motor and seal chambers whereby the motor rotor turns in a non wetted environment, or dry rotor chamber, reducing viscous drag and therefore increasing the overall efficiency of the motor. Dry rotor designs typically incorporate two mechanical seals, one located at each end of the seal chamber. The seal chamber is filled with a compatible fluid that serves to cool and lubricate the faces of the inboard seal separating the rotor chamber and the seal chamber. The outboard seal separating the seal chamber from the pumped fluid often relies on the pumped fluid for its cooling and lubrication.
When a submersible motor is immersed in fluid, pressure on the external surfaces of the motor increase proportionately with the depth of submergencexe2x80x94approximately one pound per square inch pressure for every 2.3xe2x80x2 of submergence with water. It is a well-known fact that gasses and fluids will always tend to flow from areas of high pressure towards areas of low pressure. Although submersible pumps incorporate so called xe2x80x9cmechanicalxe2x80x9d seals to prevent the ingress of fluid into the motor cavity or rotor chamber, these seals do not seal, but in actuality restrict the flow of fluid to very small levels. This fluid flow creates a hydrodynamic film on the mating seal faces that prevents overheating and premature wear.
Under normal circumstances a submersible motor always has fluid moving across the seal faces from the region of higher external pressure towards the region of lower pressure inside the motor. Manufacturers typically rely on some form of electronic moisture detectors located within the motor cavity to wam when conductive liquid has reached a level that poses a danger to the motor, or utilize stacked seals that slow the ingress of fluid sufficiently to provide a satisfactory motor life.
Past submersible designs have utilized some form of flexible device to keep the internal environment separate from but in communication with the external fluid so as to maintain a balance of pressure on the mechanical seals. These devices have taken the form of pistons, bellows, and bladders to name a few. All of these devices, although appropriate for clean environments, are unsuitable for operation in environments laden with grease, sludge, or solids that tend to defeat their movement ability.
Some designs have provided a non-submergible means for pressurizing the submergible motor through a connecting hose or the like. These reservoirs have typically been designed as separate support systems to the submersible motor and are not integral to the motor design.
U.S. Pat. No. 5,616,973, published Apr. 1, 1997, refers to a motor housing containing a plurality of integral cooling passages, through which buffer fluid is circulated by means of a co-axially mounted shaft driven vortex style impeller. The buffer fluid absorbs heat from the motor and transfers the heat into the pumped fluid via conductive heat transfer through a segregating partition that is common to both the buffer fluid and the pumped fluid. While effective at removing heat from a motor running in air, a disadvantage of this design is that, although the motor can run continuously in air, critical surfaces of the outboard mechanical seal, specifically the contacting rotating and stationary seal faces, that are subject to frictional heat build up, are located adjacent to a small annulus formed by the pump shaft and seal components, where little relative motion occurs between the buffer fluid and the critical seal surfaces.
European patent 939231A1, published Sep. 1, 1999, operates in a similar fashion utilizing an axial flow style of impeller. While effective at removing heat from a motor running in air, a disadvantage of these designs is that, although the motor can run continuously in air, critical surfaces of the outboard mechanical seal, specifically the contacting rotating and stationary seal faces, that are subject to frictional heat build up, are located adjacent to a small annulus formed by the pump shaft and seal components, wherein little relative motion occurs between the buffer fluid and the critical seal surfaces.
The buffer fluid in this stagnant zone does not provide sufficient cooling to the contacting faces of the outboard mechanical seal, which therefore must rely on the pumped fluid for cooling. In a run-dry condition where the pump has run out of fluid to pump and the motor continues to operate, or in a condition where a gas or vapor pocket surrounds the external surfaces of the outboard seal faces, overheating of the mating seal faces and subsequent premature failure of the outboard mechanical seal can occur. Therefore, the submersible motor is not capable of running for extended periods in any condition where the normally process wetted surfaces of the outboard seal faces are dry, without damage occurring to the mechanical seal faces due to heat build up. When used in a pumping application, this requires added instrumentation in the way of load sensors, level controls, and the like; or increased vigilance on the part of operators to avoid these problems. These options all have undesirable expense and reliability issues associated with them.
There have been disclosed submersible motor driven pumps with shaft seals that provide a sealing arrangement where the motor rotor chamber can be pressurized with gas from a remote source at a pressure higher than the surrounding fluid. This higher pressure is transferred into the seal chamber along the shaft thus preventing the ingress of the external fluid into the seal chamber from outside of the motor, and ingress of fluid into the motor chamber from the seal chamber.
There are, however, disadvantages in using a remote pressure source via the rotor chamber for pressurization. Mechanical seals do not xe2x80x98sealxe2x80x99, but in actuality xe2x80x98restrictxe2x80x99 flow. One disadvantage is that the seal life of this design is partially dependent on a small volume of buffer media within the seal chamber. There is no provision for supplementing buffer fluid to the seal chamber as it flows from the seal chamber to the external environment.
Another disadvantage of this design is that the volume of pressurized gas exceeds the available volume of buffer fluid to the degree that once the buffer fluid supply, within the seal chamber, is exhausted, gas will continue to pass across the seal faces. Although this will help prevent the ingress of external fluid across the seal faces, it will result in premature seal failure due to the fact that gas is not viscous enough to lubricate the seal faces of a mechanical face seal that has been designed for liquid lubrication.
Submersible motors are often oriented vertically, with the axis of the motor shaft more or less perpendicular with the earth""s surface. Gas, being lighter than liquid, tends to rise to the highest point within any containment. A common problem with submersible units is that the mechanical seal faces are often located adjacent to the highest points within the pump chamber, for the outboard mechanical seal; and also adjacent to the highest points within the seal chamber, for the inboard mechanical seal. Any gas that is present in either the pump chamber, or the seal chamber, will tend to collect at the highest point within the chambers. If the gas pocket restricts the surrounding liquid from the seal faces, overheating and premature failure of a seal may result.
The most common form of submersible application is in water as is often found in wells. A less common but increasingly more common environment for submersibles is in industrial applications where abrasive media and/or chemicals may be the submergence media. In these environments, portability of the submersible motor is a desirable feature to many users. The use of remoted reservoirs or remoted pressurization sources restricts this portability. Also, in environments where solids are present, settling of the solids in and around the heat transfer surfaces of the motor restricts the motor""s ability to transfer heat to the surrounding liquid. This results in premature motor failure.
Submersible motor driven pumps are often located in sumps, or other low areas where liquids collect, where the primary purpose is to transfer all of the collected liquid to another location. During normal operation, heat is generated within the motor due to electrical losses. This heat needs to be removed from the motor or it will build up and cause premature motor failure. Early submersible designs, relying on the superior heat transfer characteristics of fluids relative to gasses, required the motor to be submerged in liquid at all times. The primary disadvantage was that proper operation of the pump dictated that all of the liquid could not be removed from the pump site, thus defeating the primary purpose of the pump.
A number of inventions have successfully dealt with the issue of removing heat, from the motor, when the submersible motor becomes uncovered; allowing the submersible to pump the liquid down to a level below the motor. The most common designs rely on either pumping liquid through a annular chamber around the motor housing, or circulating buffer fluid, by means of a radial impeller within the seal chamber, through jackets within the motor housing, past cooling fins that transfer motor heat into the pumped fluid for cooling.
The disadvantage of these designs is that although the motor is now protected, the mechanical face seal that restricts leakage along the shaft, from the seal chamber to the external environment, relies on the pumped media for cooling. Therefore the submersible motor is not capable of operating completely dry, for extended periods, without damage. When in a pumping application this requires added instrumentation in the way of load sensors, level controls, and the like; or increased vigilance on the part of operators. These options all have undesirable expense and reliability issues associated with them.
Regardless of how well designed a piece of machinery is, all machines with moving contacting parts, are subject to wear, and require periodical preventive or repair maintenance. In an industrial environment, downtime for a piece of equipment such as a pump can often affect the overall productivity of a process, or environmental safety where effluents are involved. Submersible motors, as presently constructed, utilize shaft seals and bearings that are mounted as segregate components. Proper installation dictates the locating and fitting of these components individually, thereby prolonging the time required for assembly and disassembly.
It is an object the invention to provide a sealing arrangement for electric motor driven submersible pumps that overcomes the disadvantages noted in the prior art.
A particular object of this invention is to provide a net positive pressure in the seal chamber, relative to the external fluid, where the ingress of the external fluid between the seal faces, due to solids and other contaminants contained in the external fluid, would tend to reduce seal or motor life relative to the life obtainable with a clean, compatible, fluid between the seal faces.
Another object is to provide an environment whereby the submersible motor is able to operate dry for an extended time period, that is, to say without any contact with an external liquid for cooling purposes and without damage to the seal or motor.
A further object is to provide an environment whereby the submersible motor is able to operate dry for an extended time period, without damage to the seal or motor, regardless of the rotational direction of the motor.
Yet another object is to provide an environment, where gas that might collect, within the pump chamber, adjacent to the mating faces of the outboard mechanical seal, that will not result in dry running and resultant overheating of the mechanical seal.
Another object is to provide a seal and bearing arrangement that allows for decreased installation and removal time for the seal and bearing arrangements, as compared to that provided by present submersible designs
Another object is to provide for the above, in an environment where the external motor surfaces are exposed to liquids carrying solids and other contaminants, in such a way so as to allow the surrounding environment to carry away the heat generated by the motor without creating restrictions that might cause the solids, or contaminants, to build up, inhibiting heat transfer.
Another object is to allow for pressurization of the seal chamber with a dry rotor design so as to allow for greater operating efficiencies than provided by wet rotor designs.
Another object is to provide for portability of the equipment by providing a motor pressurization system that would form an integral part of the submersible assembly.
Another object is to allow for pressurization of the seal chamber with a dry rotor design while at the same time providing an integral reservoir to replenish the buffer medial lost during normal operation.
Other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention.